Research On Shallow Formation Velocity Estimation Method Based On The Dispersion Characteristics Of Surface Waves

Through the dispersion characteristics of surface waves,the shear wave velocity structural characteristics of the formation can be inverted,and important geological data can be provided for geotechnical investigation and engineering geology.In this paper,the multi-order mode inversion of Particle Swarm Optimization is carried out by using the full-frequency domain dispersion data combining the active source and passive source surface waves dispersion data,and the shear wave velocity structure of the experimental site is obtained.Firstly,the effects of layer thickness,shear wave velocity,longitudinal wave velocity and density on the morphology of the surface wave dispersion curve are compared and analyzed,and the main influencing factors are clarified.Secondly,the processing methods of active source and passive source surface waves are studied,and a variety of dispersion analysis methods are compared and analyzed,and reasonable parameters were set in the subsequent forward modelling to make the influence of false frequency on frequency dispersion energy imaging smaller.The influence of offset,channel spacing and arrangement length of geophone on false frequency was studied,and these parameters were better set to make the influence of false frequency on dispersion energy imaging less.Then,Particle Swarm Optimization is verified,and the theoretical model inversion research is carried out on the three-layer incremental model,the threelayer model with low-velocity interlayer and the three-layer model with high-velocity interlayer to verify the reliability and applicability of Particle Swarm Optimization inversion algorithm.Adding higher-order mode dispersion data to the surface wave fundamental mode inversion can improve the accuracy of inverting the shear wave velocity of the formation.Finally,through the real data of active source and passive source surface waves,and the corresponding frequency dispersion data is combined into full-frequency domain dispersion data,and the shear wave velocity profile is obtained by multi-order mode inversion of Particle Swarm Optimization.This method can provide important parameters for construction engineering,seismic engineering,environment,etc.,and provide important data such as regional velocity structure characteristics for geotechnical engineering investigation and engineering geology,which is of great significance for understanding the structural characteristics of the experimental site and continuously improving the surface wave exploration method.